1. Field of the Invention
The present invention relates to an electric power steering apparatus which assists force required for steering operation with turning force of a motor and to a rotary detector for detecting rotational position of the motor, particularly to a control of driving current given to the motor when a steering wheel is returned and to improvements of construction of the rotary detector.
2. Description of Related Art
An electric power steering apparatus has been developed which provides a driver a comfortable steering feeling by driving a motor for assisting steering force on the basis of a detected result of a steering torque exerted on a steering wheel and by assisting force required for steering a vehicle with turning force of the motor.
This power steering apparatus consists of a rack shaft projected in the right and left direction of a vehicle body, the both ends thereof being connected to the right and left front wheels through each tie rod respectively, and a pinion shaft engaged with the rack shaft at the middle portion of the latter, interlockingly connected with the steering wheel.
Vehicles, provided with steering mechanism of rack-pinion system for carrying out steering operation by changing rotations of the pinion accompanying the rotational operation of the steering wheel into a movement of longitudinal direction of the rack shaft, are divided broadly into the following two categories according to the set position of the motor for assisting steering force, that is, the one wherein, the shaft of above said pinion is further projected from the engaged position with the rack shaft, aforesaid motor being provided at the projected portion through an appropriate speed reducer in order to transfer the turning force, and the other wherein, an auxiliary pinion engaged with aforesaid rack shaft is provided at a position in the axial direction different from aforesaid engaged position of the pinion, aforesaid motor being provided at aforesaid auxiliary pinion through an appropriate speed reducer in order to transfer the turning force, the former being named as one-pinion system, and the latter named two-pinion system respectively according to the number of pinions engaged with the rack shaft.
In this way, in any power steering apparatus aforementioned has a problem that the steering feeling is not natural, as the turning force of the motor for assisting steering force is transferred to the extended portion of the shaft of the pinion or the auxiliary pinion through the speed reducer, thereby, when the steering wheel is returned to the position for going straight, the returning is made to be late by the moment of inertia of the motor rotor and the frictional resistance of the speed reducer.
In order to solve the problem, there is an invention disclosed in Japanese Patent Application Laid-Open No. 61-215166, 1986. This is the one wherein the motor control is carried out by acceleration and deceleration function corresponding to the rotational speed of the motor as well as the restoring force by the motor is given to the steering wheel according to the restoring force function corresponding to the size of the steering angle.
Aforesaid invention, however, has a problem that, assist force which is an essential aim of power steering is reduced, thereby assist characteristics being interfered when speedy rotation of the motor such as quick handling is required, since the acceleration and deceleration control and returning control of the motor are carried out by using rotational speed of the motor and the steering angle, whereby plugging torque becoming large particularly in high speed rotational area. And, since the restoring force is generated so that the steering wheel is returned to neutral position correspondingly to the steering angle, thereby steering feeling is not stabilized due to the vehicle speed and inertia of the motor, leading the position of the steering wheel sometimes to exceed the neutral position, whereby causing a problem that astringency of the steering being bad in the return control of the steering wheel.
On the other hand, as the turning force of the motor for assisting steering force is transferred to the projected portion of the shaft of the pinion or the auxiliary pinion, in the case where abnormality is generated in the motor for some reason, increasing rotational resistance of the output shaft thereof, thereby locking the output shaft, there sometimes is the case that steering cannot be carried out. As the conventional power steering apparatus to prevent the problems above mentioned, the one has been well known wherein an electromagnetic clutch is provided at the output shaft of the motor, and when the ignition key is ON, the motor and the speed reducer is separated by the electromagnetic clutch, then electric current is applied to the motor momentarily, thereby detecting the lock of the motor by electromotive force generated when the motor rotates by the force of inertia.
In the conventional power steering apparatus, however, as the presence of the lock of the motor is detected by electromotive force of the motor, the detection is only carried out when the ignition key is ON, thereby, in the case where the lock occurs in the motor during the running time, there is a problem that the presence of the lock is not detected. Accordingly, since the lock of the motor cannot be detected when abnormality is generated, leading to the lock of the motor, in the case where the steering mechanism cannot be steered, the conventional apparatus is not capable of coping with the situation.
In order to detect the abnormality, it is considered to install a rotary detector for detecting the rotational condition of the motor on the motor, however, there is a problem that the tachogenerator of the conventional rotary detector is not capable of detecting the low speed rotation and rotational position, and there are also problems that a rotary encoder using a photo interrupter lacks heat resistance, deteriorates with the age, and costs high.